Early Quaternary and older alluvial deposits reveal the sea level and tectonic adjustments, fold and fault lines, subsidence and uplifts that characterize the evolving Mekong River [4]. The Mekong River (Figure 1) is one of the most diverse and unique great rivers of the world with a flood pulse that drives a productive and extensive ecological system.

About 40 million years ago, its precursor, the Red River (Figure 2), drained into the sea just south of Hanoi, Vietnam. About 13 million years ago earthquakes and volcanic activity of the Himalayas re-directed the mountain drainage southward via steep gorges [5] [6]. Below this area was a wide inland sea during the Upper Mesozoic. It is likely that the ancient Mekong River flowed directly south and to the west of the Korat Upland, joining what has become the Chao Phraya River in Thailand [5]. There is evidence in Cambodia that subsidence in the Tonle Sap basin (Figure 3), during the last 12,000 years, re-aligned the Mekong River eastward, away from its former Chao Phraya connection and the Tonle Sap basin, to its present course which now flows into the South China Sea [7].

The modern day Mekong River [8] [9] carries a large amount of arsenic rich fine sediments and sands, that originated in the Himalayan and Tibetan mountainous region, to the Mekong Delta. These fluvial transported alluvial deposits have been mediated over time by glaciation, evapotranspiration and precipitation [4].

Sediments rich in soil organic carbon, such as peat, oxidize when dried. Carbon in soils (Figure 4) binds with oxygen to create carbon dioxide that is released into the atmosphere. Soils lose mass and become compacted after being deprived of carbon. In addition, organic matter is a food source for microbes,

which release carbon dioxide to the atmosphere and contribute to the soil mass loss [10].

Subsidence is a phenomenon of all deltas of the world due to natural and anthropogenic processes. These changes include coastal and river erosion, sediment compaction, tectonics, changes in land use and agricultural practices such as irrigation, mining, deforestation, groundwater removal, levees, embankments, polders and coastal zone management [11] [12]. Collectively these factors can

result in either uplift or subsidence. Net subsidence combines the effect of rising sea levels, lack of sedimentation, deposition and sinking as a result of subsidence, which can cause land loss, shoreline retreat and flooding. Anthropogenic changes include dredging, sand extraction, channeling and reservoir construction to manage and control water availability for navigation or to reduce flooding risk. Rising sea levels can magnify the consequences of land subsidence. Subsidence can increase salinization, void the efficiency of levees and other flood defenses, impact

agriculture use and adversely affect livelihoods of millions of people [11].

Subsidence can result from compaction, sedimentation tectonics and anthropogenic causes. These processes continually shape, form, reshape and reform delta land, water, and human relationships. Sea level rise and subsidence affect the quality of life, livelihoods, and availability of food for people living in the world’s deltas. Delta urbanized areas experience rapid economic growth leading to intensive ecosystem service needs. Three deltas, Ganges-Brahmaputra, Jakarta, and the Chao Phraya, have approximately 200 million people living on them and all are experiencing subsidence.

The Ganges-Brahmaputra delta southwest Bangladesh (Figure 5) was flooded in 2009 because of Cyclone Aila which displaced more than 100,000 people (Figure 6). The storm inundated the island polders and deposited fresh silt that reached depths up to 70 cm in thickness. This demonstrated the natural system still had the ability to replenish the delta. And revealed the downside of sea walls (Figure 7) and dyking huge stretches of the delta shoreline to prevent flooding. Further, dykes must be regularly raised to protect subsiding land and to keep pace with steadily rising sea levels.

A delta’s elevation above seas level depends on vertical movement resulting

from plate tectonics and other geophysical processes, aggradation, sediment compaction and the global volume of the ocean. Sea levels are rising because of natural and human induced climate change. As oceans warm, the ice sheets near the poles melt, increasing the volume of available water and resulting in sea level rise.

Dams, levees, polders, dykes and embankments that trap silt and starve deltas of new sediment severely limit aggradation. Extraction of groundwater and hydrocarbons increase sediment compaction. The total weight of urban infrastructure, such as Bangkok, Thailand [13], contributes to compaction. Groundwater recharge is reduced when pavements and nonporous structure roofs prevent surface water from percolating into the soil and underlying sediments.

Increasing compaction and reduced aggradation has put most of the world’s mega deltas in danger especially in Southeast Asia. Delta areas are sinking below sea level and the tributary and sea flooding is worsening. The biggest threat is the possibility that a mega delta will collapse. The Indus River Delta in Pakistan has already collapsed. For 140 days a year, the Indus River runs dry before reaching the Arabian Sea. The overexploited river, used for agriculture and irrigation, contributes to the river running dry. The Indus River will soon run dry during most of the year. The Indus River Delta is now only 1/10^th of its original size. Intruding seawater has displaced hundreds of thousands of people, submerged coastal villages, and contaminated adjacent groundwater reservoirs.

The Ganges-Brahmaputra Delta with 170 million people is sinking and could collapse also. Dams and levees [14] have reduced sediment delivery to

Ganges-Brahmaputra Delta. The worst subsidence has occurred in the tens of thousands of square kilometers of river islands southwest of Bangladesh. Concrete (Figure 8) and earthen embankments were built to hold back the sea [15]. Polders were built in 1960s around lower lying plots of land. The embankments blocked replenishment of the delta with river sediments carried downstream by annual monsoon floods. These islands have lost 1 - 1.5 m of elevation. Now storm surges can breach or damage the walls around the polders and create crater lakes that can last for years [16].

Jakarta pumps 180 - 250 million·m³ of groundwater per day (including legal and

illegal usage). The extraction of deep groundwater has accelerated the compaction of overlying clays in the Jakarta Delta. The trend is worse in northwest Jakarta (Figure 9) where subsidence rate is 20 cm per year. Shallow groundwater contaminated with surface pollution can remain trapped for months under the city (Figure 10) during the dry season when no water is available to flush low-lying areas.

Companies that supply treated surface water, in place of groundwater, complain they have few customers and cannot afford to make upgrades. The city’s high-rise buildings, hotels and industry prefer deep groundwater over the poor quality treated surface water. Jakarta officials suggest that if the deep groundwater use cannot be curtailed by 2030 subsidence will continue to be a problem. Either the existing population of 4 million people living in northwest Jakarta will have to evacuate to higher ground, including the entire government, or else a giant seawall (Figure 8) which is being planned by the government must be built to block the Bay of Jakarta.

Rising sea levels can magnify the consequences of land subsidence (Figure 11). In Jakarta subsidence has been rapid (Figure 12) and the experiences and lessons learned could help guide other Southeast Asia delta’s subsidence mitigation and remediation approaches.

Bangkok is located on a nearly level deltaic-marine plain (Figure 13) in 0.5 to 1.5 m above mean sea level. The pumping of groundwater in Bangkok metropolitan, during the last 50 years, has resulted in a compression of silts and surficial deposits, reduction in pore pressure, dramatic lowering of piezometric levels, and more than 0.5 m of ground subsidence (Figure 12) with a maximum settlement rate of 10 to 12 cm per year [13].

The intensive groundwater extraction is causing up to 3 cm/yr of land subsidence and a total 27 cm land surface drop between 1988 and 2020. Erban et al. [17] suggested the deep groundwater extraction is causing interbedded clays to compact and expel water containing dissolved arsenic-mobilizing solutes (e.g.) dissolved organic carbon and competing solutes to deep aquifer over decades.

The capital of Thailand, Bangkok is home of 20 million people and situated on the Chao Phraya River and Delta floodplain (Figure 13). The city is 28 km north of the Gulf of Thailand. It is known as “The Venice of the East” with dozens of large canals (Klongs) which drain into the Chao Phraya River [18]. Throughout the Bangkok area (Chao Phraya Delta), the water table is at or near the surface. Many areas in Bangkok flood quickly during the monsoon season. The city has been subsiding or sinking with some areas having already subsided 150 cm or more. Parts of the city are now below sea level. In the southeastern metropolitan

area, the floodwater resides there for much longer times and drainage has become less efficient because of reduced gradients in canals and storm drains. Artificial recharge is needed to restore piezometric heads of the multi-aquifers system.

Experts suggest that eroding shorelines and unchecked urbanization will leave Bangkok residents in a critical situation. The weight of infrastructure and people has contributed to the city’s gradual descent into the water. Bangkok has become a victim of its own developmental success. Making the situation worse is a

network of bridges and roads that have replaced the canals that are used to traverse the city. The canals were part of a natural drainage system throughout the city. Shore erosion has exacerbated loss of land and subsidence. Aquaculture development and shrimp farms have replaced the shore stabilizing mangrove forests.

Bangkok can be trapped by monsoonal tributary flooding from the north and flooding from the Gulf of Thailand to the south. In the future, more intense storms are expected occur in this region and continue to erode the shoreline and flood the city. The city weakness is a result of hyper-development of neighborhoods with small tunnels that drained water from the land. These small tunnels previously acted as water storage basins but are now inadequate due to population pressures and increasingly more frequent high precipitation events. The government is constructing a 2600 km municipal canal network with pumping stations and eight underground tunnels to mitigate the effects of potential climate change and evacuate water faster if disaster strikes. In 2017, Chulalongkorn University built a four ha park that is designed to re-direct several million liters of rainwater so surrounding neighborhoods are not flooded; however, these fixes may not be sufficient in the long term.

The Mekong River discharges 457 km³ of fresh water annually from a 795,000 km² watershed area. Discharge volume and flood timing historically have been highly predictable and concentrated in an extremely regular wet season peaks [19]. The wet or monsoon season begins when warm moist winds from the ocean blow eastward over Cambodia, Thailand, Lao PDR and Vietnam and bring heavy rainfall. The onset of flood season usually occurs near the end of June each year and lasts about 130 days. The start and end of the annual flood occurs within a period of two weeks with little variation. The dry season onset

begins in late November.

As moisture-laden winds encounter the ridges and mountains of Lao, the left-bank (eastern) tributaries of the Mekong receive high levels of monsoon rainfall. The 14 tributaries in northern Laos all drain into the Mekong River (Figure 1), including the 447 km Nam Ou River longest tributary river in Lao. Runoff from Laos tributaries of the Mekong are the source of major wet season flooding, discharge and sediment load [20]. The longest single tributary of the Mekong is the 673 km Mun River of northern Thailand. Despite its length, the Mun River contributes very little of the Mekong’s discharge and sediment load since the river drains a dry, flat region.

The Mekong Delta, with 20 million people is subsiding at the rate of 1.6 cm per year [12]. The Mekong Delta is one of the world’s major rice exporters. Delta plains have low bearing strength and soils are soft and easily compressed. These plains are usually held up above sea level by fresh groundwater that flows through the pores of sediment deposits. When those resources are extracted, such as in the Mekong Delta, the sediment compresses, the land shrinks, and the surface is lowered.